1. Field of the Invention
One aspect of the present invention is directed to a process for reducing the concentration of organosulfur compounds in any hydrocarbon-based fluid and a multi-stage system for conducting the same.
2. Discussion of the Background
Natural fuel stock comprises hydrocarbons and other undesirable components, such as organosulfur compounds. These organosulfur compounds include, but are not limited to, thiophenes, benzothiophenes, dibenzothiophenes, naphthothiophenes naphthobenzothiophenes and their substituted analogs. When combusted, these organosulfur compounds produce undesirable sulfur pollutants that have been generally attributed to societal problems such as respiratory illnesses, acid rain, etc. The sulfur pollutants also poison tail pipe catalytic converters. The catalytic converters are designed to decrease other diesel engine pollutants such as particulate matter, oxides of nitrogen and uncombusted or partially combusted hydrocarbons. Consequently, technologies have been implemented in order to remove organosulfur compounds from natural fuel stock.
At present, hydrodesulfurization (HDS) is the most commonly employed technology used to desulfurize natural fuel stock, said technology being capable of reducing the amount of sulfur to levels of about 300 to 500 ppmw (parts-per-million by weight). However, some of the above-mentioned organosulfur compounds are difficult to desulfize via HDS because they are sterically hindered. This is especially true for the 4 or 6-mono- or 4,6-di-alkyl-substituted dibenzothiophenes. Recently, newer HDS technology has been introduced that is capable of desulfurizing the “difficult to desulfurize” (or hard sulfur) compounds; consequently, this technology affords refineries with the opportunity to reduce the sulfur levels even further. However, this newer HDS technology requires more demanding desulfurization conditions, such as higher temperatures (>650° F. (343° C.)) and pressures (>1000 psig (68.9 bars)), and reduced space velocities. Under these conditions, unnecessary side reactions (e.g., hydrogenation of unsaturated carbon-carbon bonds) become kinetically viable with respect to the sulfur-reduction reaction. Accordingly, large amounts of hydrogen are required for adequate desulfurization, which in turn, results in an overall increase in operating and capital costs. This last matter is due, in part, to the fact that in order to operate at the higher temperatures and pressures, a refinery must equip itself with specialized reactors and equipment. Therefore, this newer HDS technology is somewhat cost and space prohibitive, and thus, may not be an economical alternative for many refineries.
Regardless of the economics associated with this newer HDS technology, all refineries are now facing newly promulgated governmental regulations that limit the sulfur content of fuels. Specifically, the United States Environmental Protection Agency (US EPA) will soon limit sulfur content of “on-road” diesel fuel to 15 ppmw. As noted above, this presents a problem for many refineries because the only available technology capable of producing “on-road” diesel fuel that meets this newly imposed requirement is economically unattractive.
Consequently, the newly introduced stringent regulations coupled with the shortcomings of existing HDS technology have necessitated a search for technologies that may either supplant or complement the existing HDS technology.
Ideally, it would be convenient if the organosulfur compounds could be separated from the hydrocarbon liquid by distillation. Unfortunately, this is not possible, as the physical properties of organosulfur compounds found in hydrocarbon fuels are often very similar to the fuel itself. For example, middle distillate fuels such as atmospheric or vacuum gas oils are produced via distillation. The organosulfur compounds that are contained in these gas oils have the same boiling range as the fuel itself. In fact, organosulfur compounds are found throughout the boiling range of the fuel. Therefore separation of the organosulfur compounds by distillation is not possible. However, an attractive avenue of exploration is one directed to a chemical process whereby organosulfur compounds are converted to altered organosulfur compounds whose physical properties are significantly different than those of the starting organosulfur compounds, and thus, from the overall hydrocarbon liquid.
One possible approach that has recently received attention involves oxidative desulfurization. Oxidative desulfurization operates at mild temperatures (<212° F. (100° C.)) and pressures (<30 psig (2.07 barg)), and several patents have been granted describing oxidative desulfurization processes. Some earlier U.S. Pat. Nos. (2,749,284; 3,341,448; 3,413,307), which are hereby incorporated by reference, describe two common themes of oxidative desulfurization, which include, but are not limited to, reaction of a fuel stock containing organosulfur compounds with an oxidant followed by separation. Other references (U.S. Pat. Nos. 5,753,102; 5,824,207; 5,910,440; 5,958,224; 5,961,820; 6,160,193; 6,171,478; 6,231,755; 6,254,766; 6,274,785; 6,277,271; 6,338,794; 6,402,940; 6,402,939; and 6,406,616; and US Statutory Invention Registration H1986), which are hereby incorporated by reference, encompass the earlier developed themes of oxidization of unwanted organosulfur compounds present in hydrocarbon liquids followed by separation of the oxidized organosulfur compounds from the desired hydrocarbon liquid. On the whole, these references represent the conventional processes for reducing unwanted organosulfur compounds from fuel stocks; all of which involve an oxidation reaction, wherein organosulfur compounds are converted to their respective sulfoxides and sulfones, followed by one or more separation steps. The separation steps include, but are not limited to, extraction and adsorption (either alone or in combination).
The themes associated with oxidative desulfurization contained in many of these references shows that when organosulfur compounds are oxidized, the resultant oxidized organosulfur compounds have significantly different physical properties that provide an opportunity for separating the oxidized organosulfur compounds from the hydrocarbon liquid. For example, when the sulfur-containing compounds contain thiophenic sulfur, the oxidized organosulfur compounds comprise corresponding thiophenic sulfoxides or sulfones whose physical properties (e.g., polarity and volatility) are significantly different than those of the unoxidized thiophenic compounds. These differences in the physical properties enable the separation of oxidized organosulfur compounds from the hydrocarbon fuel. Separation techniques can rely on many physical properties, and the two mentioned properties (e.g., polarity and volatility) are not exhaustive but are mentioned for illustrative purposes.
Even though the above-identified references are directed to the problem of removing unwanted organosulfur compounds from fuel stocks, these references do not adequately describe a process that may be adapted for use in middle distillate fuel stocks that contain about 5000 ppmw or more of organosulfur compounds. The reason for this lies in the overall conversion of the oxidation reaction. For example, in order to satisfy the US EPA standard of 15 ppmw, a process that includes the oxidation reaction must be able to consistently operate at a reaction conversion of no lower than about 99.4%, when the organosulfur content is about 5000 ppmw. Ideally, it is desirable to develop a substantially quantitative oxidative process, in order to remove substantially all of the sulfur-containing hydrocarbons from a middle distillate fuel stock.
Accordingly, a problem to be solved by the present invention relates to a process wherein the conversion of unoxidized organosulfur compounds to oxidized organosulfur compounds occurs substantially quantitatively. Substantially quantitative oxidation simultaneously allows for efficient separation and removal of organosulfur compounds and further recovery of hydrocarbon fuel.
This problem becomes apparent when one considers that efficiency of the above-mentioned separation processes (i.e., extraction and adsorption) is dependent upon the overall oxidation conversion process. For example, when processing fuels with approximately 5000-ppmw sulfur content, it has been found that it is advantageous to remove most of the sulfur compounds utilizing a liquid-liquid extraction process. However, an extraction step that involves high sulfur removal leads to high solvent to feed ratios. While recovery of the solvent extract after the liquid-liquid extraction does not pose major difficulties, the resultant extract is not only rich in oxidized organosulfur compounds, but also contains sulfur-free fuel components, particularly aromatic compounds. The quantity of fuel lost via the liquid-liquid extraction step may range from 20 to 35 wt %, which leads to another problem to be solved. That is, liquid-liquid extraction of an oxidized fuel stock leads to a concomitant loss of fuel. If the overall conversion of the oxidation is not substantially quantitative, then it becomes difficult to recover lost fuel. While it may be possible to further process the solvent extract stream in other refinery units or to burn the solvent extract stream for its energy value or use the solvent extract stream as an asphalt modifier, the inventors found that downgrading the solvent extract stream, i.e., as feed to another refinery processing unit, is not economically advantageous.
Accordingly, the present invention provides a solution aimed at overcoming these difficulties, by in turn providing a new process that is attractive in that it overcomes a problem of fuel loss upon liquid-liquid extraction. It is noted that minimized fuel loss is made possible by achieving substantially quantitative oxidative conversion during the oxidation stage of the overall process. Consequently, the solvent extract that contains fuel may be subjected to additional process steps that afford the recovery of fuel via distillation. This provides a higher overall recovered yield of fuel that has heretofore never been accomplished, as other oxidative processes cannot simultaneously achieve the low sulfur fuel yields made possible by the present invention.
In addition to the advantages inhered by the substantially quantitative oxidative conversion process, the present invention inheres additional advantages over pre-extraction type processes, such as those described, for example, by Gore in U.S. Pat. Nos. 6,160,193 and 6,274,785. For example, these advantages include: (1) Favors fuel recovery over minimizing oxidant consumption; (2) Minimizes the circulation of extraction solvent; (3) Eliminates the need for an extract wash step; and (4) Minimizes corrosive catalytic acids in downstream lines and equipment.